Compatible linear and branched ethylenic polymers and foams therefrom

ABSTRACT

An expanded cellular ethylenic polymer product is provided from an irradiated, noncross-linked linear ethylenic polymer. Linear ethylenic polymers can be irradiated at ambient conditions sufficient to introduce branching in the polymer in the absence of detectable cross-linking as indicated by the absence of gels. The irradiated linear ethylenic polymer is compatible with highly branched low density polyethylene and, when mixed therewith, produces a resin having a single broad based melting temperature range as determined by direct scanning calorimetry, which indicates that the polymers in the mixture have similar crystallization behavior suitable for producing low density foams by extrusion foaming. The linear polymers can be obtained from recycled shrink wrap film. Low densities of from 0.7 to less than 4 pcf can be achieved. The foams typically have improved tear resistance as compared to previous products prepared from low density polyethylene, at comparable low densities.

FIELD OF THE INVENTION

[0001] This invention relates to expanded cellular products preparedfrom polyethylene and related polymeric substances.

BACKGROUND OF THE INVENTION

[0002] Polyethylene foams prepared from low density polyethylene resins(“LDPE” resins) have been widely accepted for industrial uses.Typically, these foams have light weight and a high degree of uniformenclosed fine cells. LDPE foams can be produced with densities in therange of from about 1 to 30 pcf (16 to 480 kg/cubic meter). Polyethylenefoams generally have low water vapor transmission properties and areresistant to mechanical and chemical deterioration. Polyethylene foamsare particularly suitable for use in thermal insulation, flotation,cushioning, and packaging. LDPE resins exhibit good melt strengthdesirable for foaming by conventional methods.

[0003] LDPE is made by the so-called “high pressure” process bypolymerization of ethylene in the presence a suitable catalyst. LDPEtypically has a relatively low density of from about 0.91 g/cc to lessthan about 0.94 g/cc, typically about 0.92 g/cc. The good melt strengthof LDPE is usually attributed to the long-chain and short-chain branchesthat are distributed along and extend from the polymer backbone. Thesebranches make it more difficult for the individual molecules to slideover each other, which increases the resistance of the molten polymer tostretching during elongation. Increased resistance to stretching issometimes referred to as “extensional viscosity” and is indicative ofthe melt strength of the resin and of the ability of a polymer toproduce stable, high quality foams of low density. The cell walls formedby nucleation of bubbles during the foaming process offer sufficientresistance to expansion and do not become thin and collapse.

[0004] Efforts have been made to produce foams from so-called “linear”polyethylene resins. Generally speaking, linear resins have poor meltstrength and are considered unsuitable for making lower density foams.Whereas LDPE is relatively highly branched with widely spaced chains andcan be compared to dead tree branches piled together, linear resins arecharacterized by long, straight chains with less branching and so themolecules are more closely aligned in the manner of carefully foldedrope.

[0005] Polyethylene resins become more difficult to foam as density, andlinearity, increase. For example, unlike LDPE, high density polyethylene(“HDPE)” is produced in a low pressure process and has a relatively highdensity of from about 0.94 g/cc to 0.96 g/cc. HDPE molecules are amongthe most linear of the polyethylenes and have a small, controlled numberof short-chain branches and normally have essentially no long chainbranching. HDPE usually has a higher degree of crystallinity than LDPEand is physically a stiffer, stronger substance than LDPE. HDPE istypically about 70% crystalline at room temperature while LDPE may be aslow as about 30 to 45% crystalline. HDPE has a higher flexural modulusand increased thermal stability as compared to LDPE as indicated in partby its higher melting point, and these properties would be useful inexpanded cellular products. However, the individual molecules in an HDPEmelt can slide over each other easily, and thus HDPE generally exhibitspoor melt strength and low extensional viscosity. When HDPE polymers areused in foaming processes, these drawbacks frequently result in a largefaction of open cells, foam collapse, and process instability. The cellwalls of the HDPE foam normally do not have sufficient resistance toexpansion and become thin and collapse.

[0006] Mixing branched and linear resins has been attempted in theproduction of extruded foams and other products, including films. Unlikefilms, which can be produced from such a mixture, foams require auniform crystallization of the polymer molecules upon cooling of theexpanded resin. The different crystallization characteristics of linearand branched polyethylene resins in physical admixture typically producefoams having large voids.

[0007] There are a large number of variables that impact whether a givenresin is useful for foam production, including melt index, extensionalviscosity, the presence of a cross-linking agent, and other parameters.Good quality foams have been made in which linear polyethylenes are acomponent under certain circumstances. For example, a cross-linkingagent can be activated after extrusion to assist the foam is holding itsextruded shape.

[0008] Many methods in the art employ cross-linked polyethylenes in thefoaming process. Cross-linking enables the extruded foam to retain itsshape. For example, HDPE resin can be extruded to the desired shape,cross-linked, and then expanded, normally by a chemical blowing agentthat is activated after extrusion and cross-linking in a process calledthe “two stage process.” The resin is extruded prior to cross-linkingbecause the shape of the product is fixed after cross-linking andcross-linking strengthens the resin to withstand expansion by a blowingagent. The two-stage process is in contrast to single stage extrusionfoaming, in which a physical blowing agent, including, for example, avolatile organic compound, is mixed under pressure with a molten LDPEresin and then the mixture is extruded into a zone of lower pressure sothat the blowing agent expands upon extrusion to produce the foam.

[0009] Cross-linking can lead to gelation of the ethylene polymers,which are undesirable localized concentrations of polymer more highlycross-linked than the surrounding areas, and can decrease the meltextensibility of the polymers. As a result, foams made with cross-linkedHDPE generally have relatively high density, which is undesirable inmany applications.

[0010] Methods have also been proposed for increasing long chainbranching in the absence of cross-linking, typically by application ofradiation. These methods can require steps that increase the complexityof processing the polymer. For example, U.S. Pat. No. 5,508,319describes a process for improving strain hardening elongationalviscosity in linear polyethylene polymers such as HDPE and LLDPE in theabsence of cross-linking. The polyethylene is irradiated with highenergy ionizing radiation at a radiation absorbed dose of 2.0 megaradsor less in an environment having an oxygen content of less than 15% byvolume. The irradiated polyethylene is maintained in the environment fora period of time and is then treated to deactivate the free radicalspresent in the irradiated material. The resulting ethylene polymer issaid to have a substantial amount of long chain branches withoutcross-linking and to exhibit improved melt strength and elongationalviscosity.

[0011] U.S. Pat. No. 4,598,128 describes a method for making apolyethylene composition having enhanced temperature sensitivity andhigh low-shear viscosity. The composition is a blend of a linearpolyethylene and a long chain Y-branched polyethylene. The Y-branchedpolyethylene is prepared by irradiating a polymer comprising moleculeshaving at least one vinyl end group per molecule under non-gellingconditions in the absence of oxygen. It is disclosed that the vinyl endgroup can be created by heating an ethylene polymer under non-gelling,non-oxidizing conditions. The irradiation process is purported not tocause cross-linking.

[0012] Mobil Oil Company has recently marketed a group of HDPE resinsdesignated as the HFE-03X series that are said to have sufficient meltstrength to produce stable foams. While not wishing to be bound bytheory, the Mobil resin is believed to be a “reactor” resin that is alinear resin, but is produced with some degree of branching during thepolymerization process that is favorable for producing a foam. The MobilHFE-03X series resins are among the highest melt strength high densitypolyethylene resins available and have among the highest extensionalviscosities available. However, stable lower density foams comparable indensity to foams that can be made from LDPE, are not believed to havebeen achieved with these resins.

[0013] It would be desirable to produce stable, closed cell polyethylenefoams of the lowest possible density from linear polyethylene resins inthe absence of the drawbacks and disadvantages of complex processingsteps and special environments and to increase the available options forproducing high quality foams.

SUMMARY OF THE INVENTION

[0014] The invention is based on the recognition that linear ethylenicpolymer resins can be produced that are compatible with highly branchedlow density polyethylene resins and can be admixed therewith to producea resin having uniform crystallization behavior necessary to producestable closed cell foams of the lowest possible density. A singlemelting temperature region can be observed for an intimate admixture oflinear and branched resins, as opposed to distinct melting regions,which is indicative of uniform crystallization behavior. Quality closedcell foams, having about 80% or more of the cells closed, can beproduced at low density in the range of from about 0.7 to less than 8pcf (11 to 128 kg/cubic meter), and typically in the range of from about0.7 to less than 4 pcf (11 to 64 kg/cubic meter). Foams can be preparedin the absence of branched polyethylene having a density as low as 2 pcf(32 kg/cubic meter). Scrap materials can be recycled and used to preparethese foams.

[0015] Foams prepared from irradiated linear resins and irradiatedlinear resins blended with LDPE, in accordance with the invention, canexhibit higher flexural modulus, stiffness, and tear strength for agiven density than do foams normally obtainable from LDPE alone at thesame density. Foams from linear low density polyethylene (LLDPE) resinshow more balanced tear resistance in the machine and cross directionsthan has previously been achieved. Temperature stability is normallyimproved.

[0016] Improvements in physical properties of the foam are somewhatproportional to the amount of conventional LDPE in the blend. Forexample, up to 50% improvement in flexural modulus, which is a measureof the stiffness of the foam, can be achieved with as much as 40% ofconventional LDPE in a blend with LLDPE. However, it should berecognized that conventional LDPE can be used in the blend in greater orlesser amounts, as desired, to optimize particular properties dependingon the available resins, economic considerations, the intended use ofthe foam, and other factors.

[0017] The linear ethylenic resins useful in the practice of theinvention should have a starting melt index of at least about 0.3 to 1and are slightly irradiated in the absence of detectable cross-linkingand in the absence special processing conditions. While not wishing tobe bound by theory, it is believed that irradiation increases thebranching in the linear resins so that when these resins are mixed withbranched low density polyethylene (LDPE), then the branches becomeentangled and both temporary and permanent bonds are formed at themolecular level. Scrap shrink wrap film, which is normally a multilayerfilm and can include layers of irradiated resin, can be recycled for usein admixture with LDPE, up to about 60% by weight:of the combinedresins, to produce low density expanded cellular products at favorableeconomic conditions. Foams having enhanced or at least equivalentproperties can be produced at lower cost.

[0018] Linear polymer structures suitable for irradiation in accordancewith the invention include those polyethylenes normally considered inthe art to be linear. It should be recognized that “linear” in the artand as used herein normally means that the ethylenic polymer may exhibitsome degree of branching, usually introduced by comonomers or oligomers,although far less than “branched” low density polyethylene. For example,high pressure low density polyethylene, which is a highly branchedstructure, contains a relatively large number of both short and longchain branches, typically from about 10 to 30 per 1,000 carbon atoms.

[0019] The linear ethylenic polymers useful in the practice of theinvention can be homopolymers or copolymers of ethylene with the alkylderivatives of ethylene, which are also called alpha-olefins. Thesealpha-olefins usually have from about 3 to 20 carbon atoms in the chainand are added in relatively small amounts to modify structure, density,and crystallinity by introducing controlled branching and therebydisrupting the packing of the molecular chains. Additional linearethylenic polymers suitable for foam production in accordance with theinvention include copolymers and terpolymers of ethylene monomer oroligomer copolymerized or block polymerized with up to about 30% ofgenerally bulky monomers. These monomers are usually selected from thegroup consisting of vinyl acetate; methyl methacrylate; maleicanhydride; acrylonitrile; alpha-olefins including propylene, butylene,and methyl pentene; isoprene; styrene; acrylic acid; and ionic salts ofacrylic acid (ionomers). These various linear ethylenic polymers,copolymers, and terpolymers can be used alone or in admixture and asblends with conventional, highly branched low density polyethylene(LDPE).

[0020] The foams of the invention are prepared from linear resins thathave been irradiated to reduce their melt index by at least about 20%.Extensional viscosity is increased by at least about 200% in the absenceof cross-linking, complex processing steps, and special environments.Extensional viscosity can be increased above 250%, and typically by 350to 450%. No reduction of oxygen is required. No free radicaldeactivation step is required. For example, high density polyethylene(HDPE) and linear low density polyethylene (LLDPE) resins treated inaccordance with the invention can have extensional viscosity prior toexpansion of from about 2×10⁶ to 1×10⁷ poise at 154° C. for HDPE and at140° C. for LLDPE polymer at an extensional rate of 2 sec⁻¹ using theCogswell extensional viscosity technique mentioned in his textbookPolymer Melt Rheology by F. N. Cogswell, Woodhead Publishing Limited,Cambridge, England (1994). Linear resins having an extensional viscosityof from about 3.5×10⁶ to 4.5×10⁶ poise are somewhat more typical at thetemperatures recited above.

[0021] The linear resins can be irradiated in air at ambient conditionsof temperature and pressure and in the absence of detectablecross-linking, either before or after mixing with LDPE, if LDPE is used.The radiation dosage should be less than that threshold value thatinduces cross-linking. For example, the resin can be irradiated at 2.77Mrads (megarads) at an ambient temperature of 72° F. for about 40 to 45seconds. However, it should be recognized that the radiation dosage istemperature and time dependent and so it is not possible to meaningfullyset forth radiation dosages in the absence of a consideration of theconditions at which the radiation is applied. Normally, however, theradiation dosage will be from about 0.01 to about 4.0 Mrads at roomtemperature and pressure, for convenience, for a time sufficient toproduce resins that can be expanded to a stable low density of less thanabout 8 pcf and in the absence of detectable cross-linking of the resin.

[0022] The resins treated in accordance with the invention are suitablefor the wide variety of foaming processes known in the art, including,but not limited to, conventional extrusion foaming in which a blowingagent is mixed with molten resin under pressure and then extrudedthrough a forming die into a zone of reduced pressure. A large fractionof uniform closed cells are formed, at least about 80% of the cells, andthe foam is stable at low density. Other conventional methods forpreparing polyethylene foams should also be useful, including, forexample, two-stage expansion processes in which chemical agents areincorporated into the polyethylene resin that are capable of activationto generate a blowing agent in situ and thereby expand the resin to forma foam.

[0023] Thus, the invention provides compatibility between linearpolyethylene polymers and highly branched low density polyethylene. Aresin can be produced from an admixture of the two that exhibits asingle melting range. The invention also provides a simple and costeffective method for making foams from various linear ethylenic resinsand blends thereof and with low density polyethylene. Moreover, theexpanded cellular products can be expected to exhibit improved flexuralmodulus, stiffness, tear resistance, tensile strength, temperatureresistance, and melt strength at low densities. The foams of theinvention have enhanced performance in a broad range of applications,including packaging, automotive, and recreational applications. Ofprimary benefit, scrap shrink film and the like can be used toeconomically produce very low density foams through recycling. Densitiesof from 0.7 to less than 4 pcf ( 1 to 64 kg/cubic meter), comparable toLDPE foams, can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Some of the objects and advantages of the invention having beenstated, others will appear in connection with the accompanying graphicalrepresentations of comparative data, in which:

[0025]FIG. 1 is a plot of flexural modulus in pounds per square inchagainst foam density in pounds per cubic foot and compares conventionalfoam from branched low density polyethylene (LDPE) with foam from linearhigh density polyethylene resins (HDPE) that have been irradiated inaccordance with the invention;

[0026]FIG. 2 is a plot of machine direction tear strength in poundsforce per inch against foam density in pounds per cubic foot andcompares a curve generated for conventional LDPE foam with a curvegenerated for three foams prepared from a blend of 25% by weightirradiated Mobil HFE-034 HDPE resin treated in accordance with theinvention and 75% by weight conventional LDPE;

[0027]FIG. 3 is a plot of machine direction tear resistance in poundsforce per inch against foam density in pounds per cubic foot andcompares a curve generated for conventional LDPE foam with a curvegenerated for four foams prepared from blends of conventional LDPE andLLDPE irradiated in accordance with the invention in which the LLDPEcomponent is present in an amount of from about 60% to 100% by weight;

[0028]FIG. 4 is a bar graph showing tear resistance in pounds force perinch in the machine and cross directions for conventional LDPE foam andfor LLDPE foam prepared in accordance with the invention;

[0029]FIGS. 5 through 17 show melting points of various HDPE, LLDPE, andLDPE resins and blends thereof and are plots of temperature in degreesCentigrade against heat flow in which the data was obtained bydifferential scanning calorimetry (“DSC”), and in which:

[0030]FIG. 5 shows the melting point of conventional LDPE centeredaround 108° C.;

[0031]FIG. 6 shows the melting point of Mobil HFE-034 HDPE resincentered around 131° C.;

[0032]FIG. 7 shows the two different melting point ranges obtained for ablend of conventional LDPE and HDPE irradiated in accordance with theinvention at 30% by weight of the resin of HDPE;

[0033]FIGS. 8 through 11 show a single melting point range shifting withconcentration of irradiated HDPE in a broad DSC peak for blends of LDPEand irradiated HDPE prepared in accordance with the invention and atconcentrations of irradiated HDPE of 10% (FIG. 8), 70% (FIG. 9), 40%(FIG. 10), and 50% (FIG. 11) by weight of the resin;

[0034]FIG. 12 shows the melting point for conventional LLDPE centeredaround 124° C.;

[0035]FIGS. 13 and 14 show a single melting point range shifting withconcentration of irradiated LLDPE in a broad DSC peak for blends of LDPEand irradiated LLDPE prepared in accordance with the invention and atconcentrations of irradiated LLDPE of 10% (FIG. 13) and 50% (FIG. 14) byweight of the resin; and

[0036]FIGS. 15 and 16 show a single melting point range shifting withconcentration of a resin from recycled shrink wrap film of irradiatedLLDPE and ethylene vinyl acetate (EVA) in a broad DSC peak for blends ofLDPE and the recycled film resin prepared in accordance with theinvention and at concentrations of irradiated film resin of 10% (FIG.15) and 30% (FIG. 16) by weight of the resin.

DETAILED DESCRIPTION OF THE INVENTION

[0037] The resistance offered by a polymer when stretched is denoted asthe extensional viscosity. As a foam is expanded, nucleated bubbles growrapidly. The polymer at the cell wall is subjected to free surfaceextensional flow. Both the rate of bubble expansion and cell uniformitydepends on the resistance offered by the polymer when stretched. Thisphenomenon has been described in the textbook Polymer Melt Rheology byF. N. Cogswell, Woodhead Publishing Limited, Cambridge, England (1994).

[0038] Melt strength for foam processing can be explained in terms ofextensional viscosity behavior by a variety of experimental techniquesdescribed in the literature. Several techniques are provided in Chapter7 of the textbook Rheology by C. W. Macosko, Wiley—VCH Publishers, NewYork (1994) and in a paper published by Nelson E. Malwitz and N. S.Ramesh in the Antec 99 Conference Proceedings at pages 1177 through 1182(1999) under the title “Predicting Pressure Drop in Mixed Shear andExtensional Flow Using Converging Cone Capillary Rheology.” Tension,lubricated compression, fiber spinning, bubble collapse, and entrancepressure drop methods are examples of reported techniques used toevaluate extensional viscosity behavior and melt strength.

[0039] Extensional viscosity provides a parameter for detecting thepresence of long chain branches. Suitably high extensional viscosity isthought to be primarily responsible for enhanced foamability andenhanced melt strength for foamable polymers. In connection with theinvention, a standard published technique known as the entrance pressuredrop method was used to determine the extensional viscosity of resinsmodified in accordance with the invention. The entrance pressure dropmethod measures the tensile resistance to extension that occurs at theentrance of a capillary. The apparent extensional viscosity andextensional rate are calculated based on Cogswell's derived equationspublished in Polymer Melt Rheology.

[0040] The following Table 1 shows the increase in extensional viscosityfor Mobil HFE-034 high density polyethylene resin (HDPE) treated inaccordance with the invention. Mobil HFE-034 resin is available fromMobil Oil Company under its HFE-03X series of new high density resins.HFE-034 has a density of about 0.952 g/cc and a melt index of about 2.Generally, the bubble growth process creates strain ranging from between0.0001 to 5 sec⁻¹. A typical strain rate of 2 sec⁻¹ is considered anoptimum rate for the foam growth process and was the strain rate used inthe evaluation for Table 1. However, the qualitative trend of theexperimental data does not change at other convenient strain rates whenconsidered for comparison. TABLE 1 Extensional viscosity at 2 sec⁻¹ %Increase in extensional Resin strain rate at 154° C. (poise) viscositydue to branching Foamability Mobil HFE-034   1 × 10⁶ — Not good. Foam At154° C. collapsing. Irradiated Mobil 3.9 × 10⁶ 290%* Good for making aHFE-034 at 2.8 low density foam Mrad At 154° C.

[0041]FIG. 1 shows the flexural modulus data for irradiated 100% HDPEfoam. A 90-mm twin screw extruder was used with the standard additives,including talc as a nucleant and glycerol monostearate as a permeationmodifier. Isobutane was used as a blowing agent. The melt temperatureranged from 268 to 271° F. (404 to 406° K). The extrusion rate wastypically set at 45.4 kg/hr. The extruded foam was in the shape ofcylindrical rods.

[0042] As graphically demonstrated in FIG. 1, foam produced inaccordance with the invention has approximately double the flexuralmodulus of conventional low density polyethylene foam at comparabledensity. The foam of the invention has a flexural modulus exceeding atleast about 500 to 1,200 psi for foam density of from 1 to 5 pcf,increasingly substantially linearly with density. Flexural modulus of2,000 psi is achievable at 5 psi. The foams are stiffer and moreresistant to bending at the same force than comparable conventionalfoams. Flexural modulus is a measure of the strength of the foam, andthe difference in conventional foams and the foams of the invention arepalpable and readily apparent.

[0043] Mobil HFE-034 high density polyethylene resin was irradiated inaccordance with the invention in air at about 2.77 Mrads at 72° F. forabout 40 to 45 seconds to produce the resin that was foamed for use inthe preparation of FIG. 1. The irradiated Mobil HFE-034 resin exhibitedan increase in extensional viscosity of 410%, from 1×10⁶ poise to4.1×10⁶ poise. Typically, resins of the invention have an extensionalviscosity of from about 3.5×10⁶ to 4.5×10⁶ poise, which generally isabout a 350 to 450% increase in extensional viscosity over the sameresin prior to irradiation. Irradiation should be performed to achieveat least about a 200 to 250% increase in extensional viscosity. Therange of resins useful in accordance with the invention can have anextensional viscosity of from about 2×10⁶ to 1×10⁷ poise at, forexample, 154° C. for HDPE and 144° C. for LLDPE at an extensional rateof 2 sec⁻¹ using the Cogswell extensional viscosity technique asdiscussed above.

[0044] Other linear homopolymers of ethylene and copolymers orterpolymers of polyethylene in which ethylene is polymerized with adifferent monomer should also be useful in the practice of theinvention. These ethylenic polymers all have in common the ethylenemonomer in the backbone. While not wishing to be bound by theory, it isbelieved that linear resins having a density of from about 0.86 andhigher that are modifiable in accordance with the invention normallyshould have a preexisting relatively high extensional viscosity of atleast about 0.8×10⁶ poise and a melt index of at least 0.3 prior totreatment in accordance with the invention. These values for extensionalviscosity may indicate a certain degree of spaced long chain branches inthe resins.

[0045] Suitable resins are those polyethylenes other than LDPE andcopolymers and terpolymers prepared with ethylene monomers andoligomers. These resins include, but are not limited to, thepolyethylenes normally considered to be linear, including ethylenehomopolymer, the ethylene and alpha-olefin copolymers normallydesignated as polyethylenes, and the copolymers of ethylene with thebulkier monomers. These resins present the problem of low melt strengthand extensional viscosity and previously have been generally regarded asincompatible with LDPE and unsuitable for making stable foams. Thepolyethylene resins include high density polyethylene (HDPE), ultra-highdensity polyethylene (UHDPE), linear medium density polyethylene(LMDPE), linear low density polyethylene (LLDPE), very low densitypolyethylene (VLDPE), ultra-low density polyethylene (ULDPE),metallocene polyethylene (mPE), which is produced from metallocenecatalyzed polymerization. The alpha-olefins normally have from 3 to 20carbon atoms in the chain and include propene, 1-butene, 1-pentene,1-hexene, 1-octene, methyl pentene, and the like. Also included arevarious ethlyenic copolymers including, but not limited to, ethylenemonomers or oligomers copolymerized or block polymerized with vinylacetate (ethylene vinyl acetate or EVA); methyl methacrylate; maleicanhydride; acrylonitrile; alpha-olefins including propylene, butylene,methyl pentene; isoprene; styrene; acrylic acid; and ionic salts ofacrylic acid (ionomers). These various ethylenic polymers can be usedalone, in admixture, and as blends with conventional low densitypolyethylene (LDPE). All these ethlyenic polymers normally also areconsidered linear, excluding LDPE, even though they may contain somedegree of short and long chain branching.

[0046] LLDPE normally means a random copolymer of ethylene andalpha-olefin selected from the group consisting of C₃ to C₁₀alpha-olefins having a polymerized alpha-olefin content of about 20% byweight. Examples of suitable alpha-olefin monomers in the LLDPE resininclude, but are not limited to, butene, pentene, methyl-pentene,hexene, heptene, octene, and 1,4-butadiene. The LLDPE used in connectionwith the invention typically will be a copolymer of ethylene and butene.

[0047] Ultra-low density polyethylene normally designates linearpolymers of density from about 0.86 to 0.88 g/cc. Very low densitypolyethylene normally designates linear polymers of density from about0.88 to 0.91 g/cc. Linear low density polyethylene normally designateslinear polymers of density from about 0.91 to 0.93 g/cc. Linear mediumdensity polyethylene normally designates linear polymers of density fromabout 0.93 to 0.94 g/cc. High density polyethylene normally designateslinear polymers of density from about 0.94 to 0.96 g/cc. Ultra-highdensity polyethylene normally designates linear polymers of densitygreater than about 0.96 g/cc. Metallocene polyethylene normallydesignates linear polymers having densities of from about 0.86 to 0.95g/cc (mPE).

[0048] Suitable linear polymers have a preexisting relatively highextensional viscosity that is believed to be responsible for thesuccessful treatment of the resin to greatly increase the extensionalviscosity. No conditions of an oxygen free or oxygen reduced environmentand no free radical deactivation step are necessary in connection withthe practice of the invention. While the Mobil resin could be irradiatedunder special conditions, it is not necessary to do so to produce foamsof low density down to about 0.7 pcf. Oxygen can be present in theatmosphere in amounts greater or less than 15%, and it normally is mostconvenient and economical to irradiate the resin in the presence of air,which typically is about 21% oxygen.

[0049] The polymer resin should be uniformly exposed to high energyradiation that uniformly penetrates the polymer mass to ionize themolecular structure and to excite the atomic structure without adverselyaffecting the atomic nuclei. Irradiation of a linear polyethylenepolymer in accordance with the invention can be performed with anysuitable type of high energy radiation, including but not limited toelectron beams, gamma rays, alpha rays, beta rays, X-rays, high energyneutrons and the like. Examples of the sources of high energy radiationinclude, but are not limited to, radioactive isotopes, cathode tubes,spent fuel elements from nuclear reactors, linear accelerators, electrongenerators, and the like. Typically, electron beam radiation will beused.

[0050] It should be recognized that small amounts of linear resin thatis not irradiated can be included in the resins used in the practice ofthe invention without adversely impacting the quality of the foam.Typically, these will be high melt strength linear resins.

[0051] The irradiation step in accordance with the invention does notcause detectable cross-linking of the linear polyethylene polymer asshown by gel point analysis of foams made from the irradiated linearpolyethylene polymer. Gel point analysis indicates that the foamdissolves in decahydronapthalene without detectable gel formationaccording to ASTM test D-2765. It is believed that the resins for use inthe practice of the invention should remain substantially gel free afterirradiation, which means that the gel content should be less than about5%.

[0052] The dose and dose rate of the absorbed radiation should be chosenso that long chain branches are introduced without causing anydetectable cross-linking and to produce an extensional viscosity of atleast about 2×10⁶ to 1×10⁷ poise at previously mentioned experimentalconditions. The dose rate should be intense enough to produce thedesired increase in extensional viscosity in a reasonable amount of timein the absence of detectable cross-linking. If the dose is too low, thenthe extensional viscosity may not increase. Long chain branching and anincrease in extensional viscosity are thought to contribute to reducingthe processing heat usually associated with linear reins and to make theresin more tolerant of increased temperature. However, if the dose istoo high, then the exposure is difficult to control and cross-linkingmay result.

[0053] One rad is the quantity of radiation that results in theabsorption of 100 ergs of energy per gram of irradiated material. Theradiation can be quantitatively measured using a conventional dosimeter.The total radiation dosage should be in the range of from about 1megarad to about 4 megarads, preferably from about 1 megarad to about3.5 megarads, and more preferably from about 2 megarads to about 3megarads. The dose rate can be in the range of from about 1 megarad perminute to about 10 megarads per minute, and preferably from about 2 to 4megarads per minute.

[0054] Some minor degree of effort may be required to determine the doseand dose rate with respect to different types of linear polyethylenepolymers, different physical forms of the linear polyethylene polymers,different sources of radiation, and with respect to differentenvironments in which the irradiation is to be conducted. This effort isconsidered to be well within the capability of one skilled in the artonce apprised of the present disclosure.

[0055] The polyethylene polymer or copolymer can be in any physical formfor irradiation, solid or melt; including, but not limited to, pellets,granules, sheets, film, fine particles, and the like. Typically, thelinear polyethylene polymer is in the form of small pellets or finespherical particles, which promotes uniform irradiation.

[0056] The linear polyethylene polymer irradiated in accordance with theinvention can be subjected directly to the step of producing foams.Alternatively, the irradiated linear polyethylene polymer can be storedfor later use. Shrink wrap films that are irradiated, especially scrap,are useful sources of polymer for the practice of the invention.

[0057] All of the foam examples supporting the invention were producedby conventional extrusion foaming techniques using standard blowingagents typically used in connection with polyethylene foam production.As graphically demonstrated in FIG. 2, foams produced in accordance withthe invention have improved tear resistance in the machine direction ascompared to conventional low density polyethylene foams at comparabledensities. The foams of the invention consistently are stronger andresist tearing over a number of trials compared to conventional foam.

[0058] The LDPE foam used for the comparative examples in FIG. 2 is fromthe same production run as that used in FIG. 1. FIG. 2 shows the highertear strength exhibited by HDPE foam over the conventional LDPE foam atabout 25% HDPE by weight composition. The HDPE resin used in connectionwith FIG. 2 is Mobil HFE-034, irradiated as set forth above inconnection with FIG. 1. However, in FIG. 2, after the HFE-034 resin wasirradiated, then the irradiated resin was mixed with LDPE in an amountof 28% by weight of the irradiated resin and 72% by weight LDPE, whichwas not irradiated. A tandem extruder was used for the experiment. Theoutput rate was 106.8 kg/hr. The same additive and blowing agentcombination mentioned before in Example 1 was used. For the LDPE foam,the melt temperature ranged from 243 to 244° F. (390 to 391° K). Theextruded foam was in the shape of thin sheets. As can be seen, theimprovement in tear strength is remarkable despite the relatively highconcentration of LDPE.

[0059] It should be recognized that, for economic reasons, it typicallyis desirable to irradiate only the linear resin prior to mixing withLDPE resin. Use of scrap resin that has been previously irradiated forsome other purpose is particularly advantageous from an economicstandpoint.

[0060]FIG. 3 graphically demonstrates improvement in tear resistance forfoams made in accordance with the invention from LLDPE resin. The LLDPEresin was irradiated at about 2.77 Mrads at 72° F. for about 40 to 45seconds, as was the HDPE resin in the previous examples. Four foams wereprepared by extrusion foaming using conventional gaseous blowing agentsand techniques. A 90 mm twin screw extruder was used for theexperiments. The output rate was a 36.3 kg/hr. The same nucleating agentand blowing agent as used in previous examples were used. The foamsranged from 100% LLDPE to a blend of 60% by weight irradiated LLDPE and40% LDPE. Examples were also prepared having 85% LLDPE/15% LDPE and 72%LLDPE/28% LDPE. In all cases, the LLDPE foams and foams from blends ofLLDPE and LDPE showed marked improvement in tear resistance in themachine direction. Tear resistance should be 10 to 15 pounds force persquare inch for foam densities of from 1 to 1.5 pcf. For example, tearresistance should exceed at least about 8 lbs/in of foam at 1 pcfdensity to about 16 lbs/in at 2 pcf, increasing linearly with density.

[0061] About 60% irradiated LLDPE or HDPE in a blend with 40% LDPE isconsidered necessary to achieve a 50% improvement in physical propertiesin the foams produced, compared to conventional LDPE. However, it shouldbe recognized that substantial benefit can be obtained from practice ofthe invention in connection with blends containing more LDPE.Substantial improvement is shown in foams prepared from a blend of 25%HDPE and 75% LDPE in FIG. 2. Foams of HDPE or of LLDPE or blendsthereof, in the absence of LDPE resin, can be produced with density aslow as about 2.5 pcf. However, densities as low as about 1 pcf can beachieved by incorporating at least about 75% LDPE in the blend.

[0062] It is intended to encompass within the scope of the inventionblends that are primarily LDPE, up to about 95% by weight or more, inadmixture with the linear ethylenic resins irradiated in accordance withthe invention. Blends of the linear resins can be used in any desiredratio. However, recycled film typically should be present at less thanor equal to about 60% by weight of the resin.

[0063] Conventional LDPE foam has a substantial difference in tearresistance in the machine and cross directions and is normallyconsiderably stronger in the cross direction. As graphically illustratedin FIG. 4 for a 100% LDPE foam of density of 1.17 pef, the differencecan be as much as 5 pounds force per inch of foam. However, an exampleof 100% irradiated LLDPE foam prepared in accordance with the inventionnot only has markedly improved tear resistance in the machine direction,but has more balanced strength properties in the machine and crossdirections. The machine direction tear strength is less than the crossdirection tear strength by approximately 1 to 1.5 pounds force per inchof foam, which means that the foam has nearly equal strength in eachdirection as compared to conventional foam. Normally, tear resistance is0 to 100% greater than that of comparable conventional LDPE foam.

[0064] The thermoplastic resin composition may optionally furthercontain additives such as stabilizers, antioxidants, plasticizers,coloring agents, flame retardants, and the like.

[0065]FIGS. 5 through 16 show melting points for LDPE, HDPE, and LLDPE,and for various blends of LDPE with HDPE, HDPE and LLDPE irradiated inaccordance with the invention, and LLDPE from an irradiated shrink wrapfilm. The plots shown in these figures were all obtained by differentialscanning calorimetry, or DSC, in which the temperature and the rate ofchange of temperature is precisely determined as heat is added to orabstracted from a sample of polymer resin in a controlled constantenvironment. At phase transitions, as from solid to liquid (melting) orfrom liquid to solid (crystallization), heat is absorbed by orabstracted from a pure substance at constant temperature, producing asharp peak. The heat is recorded in Joules per gram of sample (J/g). Thescanning rate for all the plots was 10.0 degrees Centigrade per minuteand the samples weighed from about 4 to 5 mg each.

[0066] However, polymer samples typically have a range of molecularweights present, which is sometimes called polymeric diversity, and themolecular weight is directly related to the melting point. Accordingly,a sample of a single polymer having a relatively low polymeric diversitywill still exhibit a somewhat broadened peak due to the range ofmolecular weights in the sample, with the point shown as the meltingpoint for the nominal molecular weight.

[0067]FIG. 5 shows the melting point of a conventional LDPE centered at108° C. The peak begins at about 74° C. and ends at 113° C. with arelatively sharp increase at about 101° C., indicating relativelytypical low polymeric diversity in molecular weight. FIG. 6 shows a DSCplot for Mobil HFE-034 HDPE resin having a somewhat sharp peak centeredat about 131 ° C., starting at about 70° C., increasing gradually toabout 122° C., and ending at about 135° C. As is expected, the HDPE hasa somewhat higher melting point than LDPE resin. In sharp contrast toFIGS. 5 and 6, FIG. 7 clearly shows two distinct melting points for amixture of LDPE and HDPE at 30% by weight of the resin of HDPE, one forLDPE centered at about 104° C., and one for HDPE centered at about 125°C. The difference in melting points indicates clearly the difference incrystallization behavior of the two polymers, also indicating that anextruded foam product prepared from the blend of FIG. 7 would likelycollapse. However, it should be recognized that a resin blend of theinvention including linear resins may show two separate DSC peaks if thescan is taken at a very slow rate. Nevertheless, scanning at a typicalrate of about 10° C. per minute for about a 5 mg sample shows but onepeak generally and accurately reflects the molecular interaction anduniform crystal formation of blends of the invention.

[0068]FIGS. 8 through 11 show DSC plots for various blends of LDPE withHDPE that have been irradiated in accordance with the invention. In eachcase, a single peak is shown, clearly indicating intimate mixing andblending of the LDPE and HDPE components at the molecular level. Thepolymer chains of the two species have become entangled and exhibit asingle broad peak, indicating a uniform crystallization behaviorfavorable for foaming, from 10% to 40% by weight of HDPE in the resin.The melting point shifts upward as the concentration of HDPE in theblend is increased. At 10% HDPE (FIG. 8), the peak is centered at 106°C. At 20% HDPE (FIG. 9), the peak is centered at 128° C. At 30% HDPE(FIG. 10), the peak is centered at 127° C. At 40% HDPE (FIG. 11), thepeak is centered at 129° C. The melting point shift is a furtherindication of the intimate entanglement between the irradiated HDPE anLDPE and the uniform crystallization behavior of the blend.

[0069]FIG. 12 shows the DSC plot for LLDPE having a melting pointcentered at about 124° C., which is higher than LDPE, as expected. FIGS.13 and 14 show DSC plots for blends of LDPE and LLDPE irradiated inaccordance with the invention in concentrations of from 10% to 50% byweight of the resin, respectively. The results are similar to thoseshown in FIGS. 8 through 11 for blends of LDPE and irradiated HDPE, inthat a single broad peak that shifts with concentration of LLDPE isshown.

[0070] FIGS. 15 an 16 are similar to the DSC plots of FIGS. 13 and 14 inthat they show DSC plots for blends of LDPE and LLDPE in accordance withthe invention. However, the LLDPE of FIGS. 13 and 14 is obtained byrecycling an irradiated shrink film. Densities of from 0.7 to less thanabout 7 pcf (11 to 112 kg/cubic meter) can be achieved for foamsproduced from recycled shrink film. Shrink film that is irradiated andsubstantially gel free provides a convenient source of resin having areduced melt index. Typically, the melt index has been reduced by 20 toabout 98% or more. Polyethylenic films that have not been irradiatedshould be subjected to irradiation in accordance with the invention,either before or after preparation for use in an extruder or otherfoaming means.

[0071] The resin from the film is used to obtain the plots of FIGS. 15and 16 in concentrations of from 10% to 30% by weight of the resin,respectively, although up to 60% of the resin can be used to producegood quality foam from recycled film. A typical resin at 60%concentration comprises about 40% by weight LDPE and 44% LLDPE, 8% EVA,and 8% LMDPE. The results are similar to those shown in FIGS. 13 and 14for blends of LDPE and irradiated LLDPE, in that a single broad peak isshown. Other ethylenic polymer films should be useful for the practiceof the invention. These films may be prepared from ethylenic polymersselected from polyethylenes, including LDPE and linear polyethylenes;copolymers or terpolymers of ethylene monomers or oligomers and one ormore alpha-olefins; copolymers and terpolymers of ethylene monomers oroligomers and monomers or oligomers selected from the group consistingof vinyl acetate, methyl methacrylate, maleic anhydride, acrylonitrile,isoprene, styrene, acrylic acid, and ionic salts of acrylic acid; andblends of one or more thereof.

[0072] The polyethylene resins include high density polyethylene (HDPE),ultra-high density polyethylene (UHDPE), linear medium densitypolyethylene (LMDPE), linear low density polyethylene (LLDPE), very lowdensity polyethylene (VLDPE), ultra-low density polyethylene (ULDPE),metallocene polyethylene (mPE), which is produced from metallocenecatalyzed polymerization. The alpha-olefins normally have from 3 to 20carbon atoms in the chain and include propene, 1-butene, 1-pentene,1-hexene, 1-octene, methyl pentene, and the like. Also included arevarious ethlyenic copolymers including, but not limited to, ethylenemonomers or oligomers copolymerized or block polymerized with vinylacetate (ethylene vinyl acetate or EVA); methyl methacrylate; maleicanhydride; acrylonitrile; alpha-olefins including propylene, butylene,methyl pentene; isoprene; styrene; acrylic acid; and ionic salts ofacrylic acid (ionomers).

[0073] Recycled film and its use in the preparation of foams isdiscussed in the following Examples.

EXAMPLES

[0074] The following examples illustrate the use of the recycled shrinkwrap film discussed in connection with FIGS. 15 and 16 as the source ofirradiated LLDPE polymer for use in the practice of the invention. Therecycled shrink film had a three-layer coextruded composite structure,containing 74% linear low density polyethylene (LLDPE), 13% linearmedium density polyethylene (LMDPE), and 12.4% ethylene vinyl acetate(EVA): LLDPE+LMDPE+EVA/LLDPE/LLDPE+LMDPE+EVA. This film is described inU.S. Pat. Nos. 4,551,380 and 4,643,943. The film had previously beenirradiated at 2.3 to 3.0 Mrads. Before irradiation, the coextruded filmhad an overall melt index of 1.0 and a heavy weight melt index of 33.9under a weight of 21.6 kg. After the film was irradiated, the film hadan overall melt index of less than 0.1 and a heavy weight melt index of1.3 under the same weight. The film was pelletized to facilitate feedingto foam extruder. It is called “shrink film” in the following examples.

Example 1

[0075] Foam sheet was prepared on a tandem extrusion system having afirst or primary extruder of 11.4 cm diameter and a second, orsecondary, extruder of 15.2 cm diameter. The blowing agent was propane.Talc and fatty acid (glycerol monostearate) were added for control ofcell nucleation and dimensional stability. The results comparing 100%LDPE foam and various proportions of resin from shrink film added to theLDPE resin in amounts of from 15 to 50% by weight of the resin arepresented in Table 1 below: TABLE 1 1 2 3 4 5 LDPE, wt % 100 85 75 65 50Shrink Film wt % 0 15 25 35 50 Total rate Kg/hr 312 321 300 295 —Propane Kg/hr 35.5 39 41 — 41 Melt T. ° C. 108 113 115 114 — Die P. Mpa9 8.4 8.0 7.5 7.8 Thickness, mm 3.5 3.2 2.9 3.1 2.6 Density, Kg/m³ 18 2122 18 22 Cells, MD #/cm 12 10 11 12 14 TD 11 11 12 13 16 Tear Strength7.4 11.3 13 9 14 lb/in, MD TD 11 13 14 13 17 Tensile, lb/in² MD 52 53 6149 67 TD 28 36 38 34 44 Puncture, lb 6.3 7.0 7.2 7.2 7.0

[0076] As is shown from the table above, tear strength, tensile, andpuncture resistance of the thin sheet extruded foam prepared from amixture of LDPE and LLDPE from recycled shrink wrap film were comparableor increased over that for foam of comparable thickness and densityprepared from a 100% LDPE resin.

Example 2

[0077] Foam sheet was prepared on a tandem extrusion system having afirst extruder of 8.9 cm diameter and a second extruder of 11.4 cmdiameter. The blowing agent was propane. Talc and fatty acid (glycerolmonostearate) were added for control of cell nucleation and dimensionalstability. The total resin flow rate was 248 kg/hr. The resultscomparing 100% LDPE foam and various proportions of resin from shrinkfilm added to the LDPE resin in amounts of from 5 to 20% by weight ofthe resin are presented in Table 2 below. TABLE 2 1 2 3 4 LDPE wt % 10085 85 80 Shrink film wt % 0 5 15 20 Propane — 35.5 36.8 38.2 Melt T. °C. 110 110 112 113 Die P. MPa — 5.9 6.1 5.5 Thickness, mm 2.6 2.5 7.52.4 Density, Kg/m³ 17.6 17.6 19.2 19.2 Tear Strength 5.6 7 8 8 lb/In, MDTD 10 10 11 12

[0078] Table 2 shows comparable or improved tear strength for thin sheetextruded foam prepared from a mixture of LDPE and LLDPE from recycledshrink wrap film when compared to foam from 100% LDPE resin of similarthickness and density.

Example 3

[0079] Foam rod was prepared on a 90 mm diameter counter-rotating twinscrew extruder with a rod die. The blowing agent was isobutane. Thepressure was taken right before the die. The results comparing 100% LDPEfoam and various proportions of resin from shrink film added to the LDPEresin are presented in Table 3 below. TABLE 3 Shrink LDPE Film ButaneScrew Pressure Melt T. Rod Dia. Density Kg/hr kg/hr kg/hr RPM MPa ° C.cm kg/m³ 1. 68.2 — 6.6 34 3.46 112.8 7.6 28.6 2. 64.8 3.4 6.4 44 2.72117.2 6.8 32.2 3. 61.4 6.8 6.4 44 — 120 — 37.3 4. 54.6 13.6 6.4 40 3.88122.2 6.8 37.3 5. 47.8 20.4 6.5 35 5.17 127.2 6.8 30.1 6. 37.5 30.7 6.535 5.92 121.1 6.7 30.2 7.* 28.6 39.6 6.5 37 6.19 123.3 6.1 33

[0080] Table 3 shows that foam rods of comparable density were preparedfrom 100% LDPE resin and from LDPE resin having various amounts of LLDPEresin from shrink film admixed therewith.

[0081] Although the foregoing invention has been described in somedetail by way of illustration and example for purposes of clarity ofunderstanding, and although specific terms have been employed, theseterms have been used in a generic and descriptive sense only and not forpurposes of limitation.

What is claimed is:
 1. A foam product having a density of from about 0.7to less than 8 pcf (about 11 to 128 kg/cubic meter) comprising one ormore ethylenic polymers selected from the group consisting ofpolyethylenes other than low density polyethylene (LDPE); copolymers orterpolymers of ethylene monomers or oligomers and one or morealpha-olefins; copolymers and terpolymers of ethylene monomers oroligomers and monomers or oligomers selected from the group consistingof vinyl acetate, methyl methacrylate, maleic anhydride, acrylonitrile,isoprene, styrene, acrylic acid, and ionic salts of acrylic acid; blendsof one or more thereof; and blends of one or more thereof with LDPE,wherein said ethylenic polymers other than LDPE are irradiated.
 2. Theproduct of claim 1 wherein said ethylenic polymers are selected from thegroup consisting of ultra high density polyethylene (UHDPE), highdensity polyethylene (HDPE), linear low density polyethylene (LLDPE),linear medium density polyethylene (LMDPE), very low densitypolyethylene (VLDPE), ultra low density polyethylene (ULDPE), andmetallocene polyethylene (mPE), and ethylene vinyl acetate copolymer. 3.The product of claim 1 wherein said irradiated ethylenic polymer issubstantially gel free.
 4. The product of claim 3 wherein saidalpha-olefin is selected from the group consisting of propene, 1-butene,1-pentene, 1-hexene, 1-octene, methyl pentene, propylene, butylene, andmethyl pentene.
 5. The product of claim 1 wherein said irradiatedethylenic polymer is compatible with LDPE and in admixture therewithexhibits a single melting temperature range for each blend as determinedby differential scanning calorimetry at a sample rate of about 10° C.per minute for about a 5 mg sample.
 6. The product of claim 1 whereinsaid irradiated ethylenic polymer has a melt index of at least about 20%less than the same polymer prior to irradiation.
 7. The product of claim1 wherein said irradiated ethylenic polymer has an extensional viscosityat least about 200% greater than the same polymer prior to irradiation.8. The product of claim 1 wherein said irradiated ethylenic polymer hasan extensional viscosity at least about 250% greater than the samepolymer prior to irradiation.
 9. The product of claim 1 wherein saidirradiated ethylenic polymer has an extensional viscosity at least about350% greater than the same polymer prior to irradiation.
 10. The productof claim 1 wherein said blend including LDPE is irradiated.
 11. Theproduct of claim 1 wherein said ethylenic polymers are irradiated in anatmosphere in which the active oxygen concentration is at least 15% byvolume of the atmosphere.
 12. The product of claim 1 wherein saidethylenic polymers are irradiated in air at ambient temperatureconditions.
 13. The product of claim 1 wherein said product has aflexural modulus exceeding at least about 500 psi at 2 pcf to about1,200 psi at 5 pcf, said flexural modulus increasing substantiallylinearly with density.
 14. The product of claim 1 wherein the tearresistance in the cross direction is within 25% of the tear resistancein the machine direction.
 15. The product of claim 14 wherein saidethylenic polymer is linear low density polyethylene.
 16. The product ofclaim 1 wherein the tear resistance in the machine direction exceedsfrom about 8 lbs/in of foam at 1 pcf to about 16 lbs/in at 2 pcf andincreases linearly with density.
 17. The product of claim 1 wherein theproduct has a tear strength at a given density that is from 10 to 100%greater than the tear strength of a closed cell expanded polyethylene ofthe same density consisting of LDPE.
 18. The product of claim 1 furthercomprising ethylenic polymers other than LDPE that have not beenirradiated prior to foaming.
 19. The product of claim 1 wherein saidethylenic polymer has a melt index of at least 0.3.
 20. The product ofclaim 1 wherein said density is from about 0.7 to 6 pcf ( 1 to 96kg/cubic meter).
 21. The product of claim 1 wherein said density is fromabout 0.7 to less than 4 pcf (11 to 64 kg/cubic meter).
 22. The productof claim 1 wherein said foam is prepared from one or more ethylenicpolymers in the absence of LDPE and said density is from about 2 pcf toless than 4 pcf (32 to 64 kg/cubic meter).
 23. The product of claim 1wherein said product is a blend of from about 5 to 100% by weightirradiated HDPE and from 0 to 95% by weight LDPE.
 24. The product ofclaim 1 wherein said product is a blend of LDPE and a irradiatedpolyethylene film comprising linear low density polyethylene (LLDPE),which film is present in an amount of about 60% by weight or less of theblend.
 25. The product of claim 24 wherein said film also comprisesethylenic polymers selected from the group consisting of polyethylenes;copolymers or terpolymers of ethylene monomers or oligomers and one ormore alpha-olefins; copolymers and terpolymers of ethylene monomers oroligomers and monomers or oligomers selected from the group consistingof vinyl acetate, methyl methacrylate, maleic anhydride, acrylonitrile,isoprene, styrene, acrylic acid, and ionic salts of acrylic acid; andblends of one or more thereof.
 26. The product of claim 24 wherein saidfilm is pelletized.
 27. The product of claim 1 wherein said ethylenicpolymer is ethylene vinyl acetate.
 28. The product of claim 1 whereinsaid product is a single stage extruded closed-cell foam having at leastabout 80% closed cells.
 29. A foam product comprising a blend of lowdensity polyethylene (LDPE) and resin from polyethylenic film whereinsaid film resin is present in an amount of from about 60% by weight orless of the blend, and has been irradiated, and wherein the product hasa density of from about 0.7 to 7 pcf.
 30. The product of claim 29further comprising, in admixture, one or more ethylenic polymers fromsaid shrink film selected from the group consisting of polyethylenes;copolymers or terpolymers of ethylene monomers or oligomers and one ormore alpha-olefins; copolymers and terpolymers of ethylene monomers oroligomers and monomers or oligomers selected from the group consistingof vinyl acetate, methyl methacrylate, maleic anhydride, acrylonitrile,isoprene, styrene, acrylic acid, and ionic salts of acrylic acid; andblends of one or more thereof.
 31. The product of claim 29 wherein LDPEis present in an amount of at least 40% by weight, and linear lowdensity polyethylene is present in an amount of about 44% by weight, andthe foam product further comprises linear medium density polyethylenepresent in an amount of about 8% by weight and ethylene vinyl acetatepresent in an amount of about 8% by weight based on the total weight ofthe blend.
 32. The product of claim 29 wherein the melt index of thecomponents of the film, in admixture, is reduced from about 20 to 98% byirradiation.
 33. The product of claim 29 wherein the density is from 0.7to 3 pcf.
 34. A foam product having a density of from about 0.7 to lessthan 4 pcf (about 11 to 64 kg/cubic meter) comprising one or moreirradiated linear ethylenic polymers blended with low densitypolyethylene.
 35. The foam product of claim 34 wherein said linearethylenic polymers are selected from the group consisting ofhomopolymers of polyethylene; copolymers or terpolymers of ethylenemonomers or oligomers and one or more alpha-olefins; copolymers andterpolymers of ethylene monomers or oligomers and monomers or oligomersselected from the group consisting of vinyl acetate, methylmethacrylate, maleic anhydride, acrylonitrile, isoprene, styrene,acrylic acid, and ionic salts of acrylic acid; and blends of one or morethereof.
 36. The foam product of claim 34 wherein said linear ethylenicpolymers have branches resulting from irradiation and, in the case ofcopolymers and terpolymers, from the presence of nonethylenic monomersor oligomers.
 37. A method for making an expanded cellular product, saidmethod comprising the steps of: (a) irradiating one or more ethylenicpolymers other than low density polyethylene (LDPE) under conditionssufficient to reduce the melt index of the polymer by at least about 20%in the substantial absence of the formation of gels; (b) forming athermoplastic resin composition comprising said ethylenic polymer and afoaming agent; and (c) expanding the thermoplastic resin compositioninto a cellular product having a density of from about 0.7 to less thanabout 8 pcf.
 38. The method of claim 37 wherein said irradiation occursin an environment in which the active oxygen concentration is at least15% by volume of the environment.
 39. The method of claim 37 whereinsaid irradiation occurs in air at ambient temperature.
 40. The method ofclaim 37 wherein said ethylenic polymer other than LDPE is irradiated ata dosage of from about 1 to about 4.0 Mrad at a rate of from about 1 to10 Mrads per minute.
 41. The method of claim 37 wherein said ethylenicpolymer is irradiated at a dosage of 2.77 Mrad for 40 to 45 seconds at72° F.
 42. The method of claim 37 wherein said product has a density offrom about 0.7 to less than 4 pcf.
 43. The method of claim 37 furthercomprising the step of mixing said ethylenic polymer with low densitypolyethylene either before or after the irradiating step, and whereinsaid ethylenic polymer is selected from the group consisting ofpolyethylenes other than low density polyethylene (LDPE); copolymers orterpolymers of ethylene monomers or oligomers and one or morealpha-olefins; copolymers and terpolymers of ethylene monomers oroligomers and monomers or oligomers selected from the group consistingof vinyl acetate, methyl methacrylate, maleic anhydride, acrylonitrile,isoprene, styrene, acrylic acid, and ionic salts of acrylic acid; blendsof one or more thereof; and blends of one or more thereof with LDPE,wherein said ethylenic polymers other than LDPE are irradiated andsubstantially gel free.
 44. A method for making a foam having a densityof from about 0.7 to less than 4 pcf comprising the steps of treating alinear ethylenic polymer to increase the branching thereof, mixing thelinear ethylenic polymer with low density polyethylene, and expandingthe mixture to form the foam.
 45. The method of claim 44 wherein thestep of treating is irradiating in the substantial absence of gels. 46.The method of claim 44 wherein the mixture exhibits a single meltingtemperature range as determined by differential scanning calorimetry ata sample rate of about 10° C. per minute for about a 5 mg sample.
 47. Amethod for making an expanded cellular polyethylene product, said methodcomprising the steps of: forming a thermoplastic resin compositioncomprising low density polyethylene (LDPE) and a shrink film comprisingsubstantially gel-free irradiated linear low density polyethylene(LLDPE), wherein the LDPE is present in the resin in an amount of atleast about 40% by weight of the resin; and expanding the thermoplasticresin composition into a cellular product.
 48. The method of claim 47wherein said step of expanding comprises mixing a volatile blowing agentwith said resin under pressure and then extruding the resin into aregion of reduced pressure to expand said blowing agent and grow saidcellular product.
 49. The method of claim 47 further comprising thesteps of plastifying said shrink film and pelletizing said shrink film.50. The method of claim 49 wherein said shrink film also comprisesethylene vinyl acetate.